Polyethylene (PE) is synthesized by polymerizing ethylene (CH2═CH2) monomers. Because PE is cheap, safe, stable to most environments and easy to be processed polyethylene polymers are useful in many applications. According to the properties polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
Ethylene polymerizations are frequently carried out in a loop reactor using monomer, liquid diluent and catalyst, one or more optional co-monomer(s), and hydrogen. The polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in a form of solid particles, which are suspended in the diluent. The slurry in the reactor is circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. The product is discharged from the loop reactor by means of settling legs, which operate on a batch principle to recover the product. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product is further discharged through flash lines to a flash tank, where most of the diluent and unreacted monomers are flashed off and recycled. The polymer particles are dried, additives can be added and finally the polymer may be extruded and pelletized.
Molecular properties of ethylene polymers, such as crystallinity or density, average molecular weight and molecular weight distribution (MWD) are determined by a number of factors such as nature and concentration of the reactants or polymerization conditions. The molecular weight distribution (MWD), also referred to as polydispersity, is defined as the ratio of weight average molar mass (Mw) divided by number average molar mass (Mn). MWD gives an indication of the uniformity of the degree of polymerization and thus the length and weight of the polymer chains. Thus, polymers with a lower MWD are characterized by higher uniformity than polymers with a higher MWD. In general, polyethylene polymers with narrow molecular weight distributions have greater stress cracking resistance and better optical properties. Polyethylene polymers with broad molecular weight distributions generally have greater impact strength.
Control of reactions conditions during the polymerization process for controlling the molecular weight distribution of a polymer is important but is difficult to realize in practice. During the polymerization process the concentration of reactants, including monomer, one or more optional co-monomer(s), and hydrogen, will vary and tend to decrease as the reactants are converted to form polymer in the course of the polymerization process. While reactants are depleted along the path of the reactor, fluctuations in reaction temperatures and fluctuations in reactant concentration occur along the reactor. As the length of the loop reactor increases, the concentration of reactants will tend to vary to a greater extent. The degree to which the concentration of reactants diminishes is also dependent upon the velocity of the corresponding reactions. The variation in concentration of a reactant will be more pronounced the faster the reactant is consumed during the polymerization process. This will contribute to non-uniform polymer properties. Especially hydrogen is difficult to control since the concentration of hydrogen required during polymerization is very small.
WO 2004/024782 discloses a slurry polymerization process wherein olefin monomer is fed to a continuous loop reactor at two or more feed points. This document also refers to the possibility of introducing catalyst in the loop reactor through a plurality of catalyst feeds. Further, in accordance with this document, a plurality of co-monomer feeds may be provided on the loop reactor. However, this document remains silent regarding the need of controlling the concentration of hydrogen in the polymerization reactor during polymerization.
In view of the above, there remains a need in the art to provide a process for improving the polymerization of ethylene. It is therefore an object of the present invention to provide an improved ethylene polymerization process.
More in particular, it is an object of the invention to provide a process for controlling the molecular weight distribution of ethylene polymer. It is in particular an object to provide a process for narrowing the molecular weight distribution of ethylene polymer.
It is a further object of the present invention to provide a process for controlling the concentration of reactants, and especially of hydrogen, in a polymerization reactor during polymerization.
The present invention aims to provide a polymer end product having improved compositional homogeneity. The present invention also aims to provide a polymer end product having improved quality. It is another object of the invention to provide a polymer end product having improved physical optical properties.